1. Field of the Invention
The invention relates to a method for utilizing thermal energy contained in subterranean formations to produce mechanical energy, and particularly to an improved method for utilizing the heat content of geothermal brine to generate electricity.
2. Description of the Prior Art
Sources of geothermal energy are found in many parts of the world, and while a few of these sources provide dry steam which is particularly useful for the production of useful energy, and some may provide wet steam, the remaining sources provide only hot geothermal brine. Because geothermal brines contain significant amounts of dissolved salts and suspended solids, past attempts to utilize these brines in the production of useful energy have been only of limited success due to the problems of scaling, plugging and corrosion of process equipment.
In one method for utilizing geothermal brine to produce mechanical energy, the hot geothermal brine is heat exchanged with a working fluid in conventional indirect heat exchange equipment in which the brine and working fluid directly contact the heat exchange surfaces of the heat exchanger; however, due to direct contact of the brine, extensive corrosion and fouling of the heat exchanger will result. Another method, disclosed in U.S. Pat. No. 3,988,895 to Sheinbaum, involves direct heat exchange of a working fluid with the hot brine. The working fluid is vaporized and the vapor is fed to a turbine for the production of mechanical energy. Still another method, disclosed by Sheinbaum and U.S. Pat. No. 3,845,627 to Hutchinson, involves direct heat exchange of a heat transfer fluid (HTF) with the brine. The hot HTF is separated from the brine and in an associated heat exchanger the hot HTF is employed to heat a working fluid by indirect heat exchange. The heated working fluid is subsequently fed to a heat engine for the production of mechanical energy. These latter methods have the disadvantage that the hot HTF removed from the direct exchanger will unavoidably carry entrained salts and other solids to the turbine or associated indirect heat exchangers, thereby causing scaling and corrosion. Also, the residual brine will still contain a large amount of unrecovered thermal energy; and furthermore, since an appreciable amount of steam is produced in the direct contact exchanger, the salt and solids content of the residual brine will increase, and as the brine temperature is reduced, the solubilities will decrease, causing more precipitation, scaling and corrosion.
Thus, there is a need for a method of producing mechanical energy which utilizes geothermal brine, wherein the problems of scaling, corrosion and precipitation associated with the prior art methods are avoided.
A device that has been successfully employed in the desalination of sea water is the multistage flash heat exchanger. U.S. Pat. No. 3,972,193 to T. K. Sherwood discloses the use of a multistage flash heat exchanger to produce mechanical energy from geothermal brine. In this method the brine is flowed through a series of flash stages which are maintained at successively lower pressures so that the brine is partially flashed to vapor in each stage, a working fluid is countercurrently flowed through the series of stages in indirect heat exchange with the water vapor produced in each stage so that the water vapor is condensed in each flash stage and the working fluid is progressively heated, and the heated working fluid is used in a heat engine to produce mechanical energy. It has been found that although this method is successful in reducing the amount of scaling, corrosion and precipitation as compared to the other prior art methods, the application of this method is restricted due to the use of flashed steam as the heat transfer medium by the flashing characteristics of the geothermal brine and is accordingly relatively inflexible.
Accordingly, a principal object of this invention is to provide a method for producing energy from geothermal brine without the deleterious effects of corrosion, scaling and plugging due to suspended solids and dissolved salts and which can be applied to a wide variety of geothermal brines.
Another object of this invention is to provide a method for producing mechanical energy from geothermal brine which does not require the brine to contact a heat exchange surface.
Yet another object of this invention is to provide a flexible method for recovering the thermal energy contained in hot geothermal brine and utilizing the recovered thermal energy to produce mechanical energy.
A further object of this invention is to provide a method for producing mechanical energy from geothermal brine, in which brine-produced vapors are not fed to the heat engine.
A still further object of this invention is to provide a flexible method which through proper choice of a heat transfer fluid, a working fluid and operating conditions can be successfully utilized to produce mechanical energy from a wide variety of geothermal brines.
Further objects and advantages of this invention will be apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.